A central problem in the development and function of epithelial cells is the process by which specialized membrane domains are formed and maintained. These domains, including the apical and basolateral membranes, the junctional complex, and other subdomains, are important in organizing and compartmentalizing membrane-related functions. Intercellular junctions, including adherens, tight, and septate junctions, have well-defined structural functions in intercellular adhesion and preventing paracellular flow. In addition, there is an increasing body of evidence that these junctions, and other specialized membrane domains, play an essential role in regulating signaling pathways. The overall goal of this proposal is to better understand how specialized membrane domains, including intercellular junctions and signaling complexes, are established and regulated in developing epithelial cells. More specifically, we plan to elucidate the functions of the epithelial septate junction, a key part of the junctional complex that forms between Drosophila epithelial cells. Septate junctions are structurally and molecularly homologous to the mammalian paranodal junction that separates the node of Ranvier from the myelinated portion of the axon. Studies in our laboratory and others have identified a handful of septate junction components, but we still have a very limited understanding of either its structure or function. To shed light on these important questions, we plan to: 1) Identify novel septate junction components using a genetic and cellular approaches. 2) Study the functions of Ankyrin, a membrane associated protein that is believed to interact with some septate junction components, in organizing the basolateral membrane. 3) Examine functional interactions between the septate junction and mechanisms that establish and maintain apical/basal epithelial polarity. These experiments are expected to provide insights into questions of fundamental importance in all developing tissues and in particular epithelia. Knowing more about the mechanisms by which epithelial cells establish and maintain contacts, regulate paracellular diffusion, and form specialized membrane domains is important not only for understanding normal physiology and development, but also for elucidating processes associated with a variety of human diseases.